Web services consumption and calling

Web services consumption and calling

I want to ask how can I add web service into my already existing MVC 4.0
project? I have searched on Internet, in all the links they are saying to
create a new application/solution but I want to add a web service and how
It will be available to call from other web sites. Can somebody please
tell me the steps for it. I will be thankful to you. I just have basic
knowledge of .Net.
2)Suppose I have to call a web service from another site so how will I
able to call that service if I do not have my own?
I have searched following links:
http://www.aspdotnet-suresh.com/2011/05/aspnet-web-service-or-creating-and.html
http://www.codeproject.com/Articles/337535/Understanding-the-Basics-of-Web-Service-in-ASP-NET